1. Field of the Invention
The present invention relates to elastic wave devices, and, more particularly, to an elastic wave device that uses a surface acoustic wave or a boundary acoustic wave propagating on a piezoelectric substrate.
2. Description of the Related Art
An elastic wave device excites a surface acoustic wave or a boundary acoustic wave using a comb-shaped electrode (Interdigital Transducer (IDT)) formed on a piezoelectric substrate having piezoelectricity. At that time, a bulk wave that includes unnecessary oscillations generated at the IDT may be reflected from the undersurface of the piezoelectric substrate, be returned to the surface of the piezoelectric substrate, and be received by the IDT. In this case, a ripple is generated in a frequency characteristic.
In order to prevent such an occurrence, a method of roughening the undersurface of a piezoelectric substrate is known.
For example, as illustrated in a cross-sectional view in FIG. 8, in a surface acoustic wave filter in which input-side electrodes 120a and 120b and output-side electrodes 122a and 122b are formed on the surface of a piezoelectric substrate 110, a rough surface area 114 is provided inside a margin frame 110a of an outer periphery of an undersurface of the piezoelectric substrate 110. As a result, a bulk wave is irregularly reflected in the area 114 on the undersurface, and it is possible to suppress reaching of the bulk wave to the output-side electrodes 122a and 122b. This suppresses the deterioration of a frequency characteristic. See, for example, Japanese Unexamined Patent Application Publication No. 2003-8396.
Unless the undersurface of a piezoelectric substrate is roughened, a ripple that is impermissible will be generated in a filter characteristic. However, when the undersurface of a piezoelectric substrate is roughened, the piezoelectric substrate is easily cracked from a microcrack on the undersurface of the piezoelectric substrate. This leads to the deterioration of a flexural strength. When a piezoelectric substrate is reduced in thickness to miniaturize an elastic wave device, the deterioration of a frequency characteristic caused by a bulk wave is increased. Furthermore, the thinner a piezoelectric substrate, the easier it cracks. Accordingly, it is difficult to suppress the deterioration of a frequency characteristic by roughening the undersurface of a piezoelectric substrate.